As described in more detail below, prior art washing machines characteristically include dish rack baskets which are carried on a moving belt, the baskets first being transported beneath a washing spray and then under a rinsing spray. The baskets moved along a path above a trough that contains the washing bath, which is generally heated and thereafter suctioned to the washing spray station. The washing bath liquid suctioned by the washing spray falls back into the bath after contacting the dishes, and the rinse water falls likewise into the bath. An overflow arrangement keeps the bath in the trough at a constant maximum level. The detergent concentration in the bath will, of course, diminish because of dilution caused by the addition of rinse water falling into the trough. To re-establish this concentration to its initial value (and thereby ensure washing which is of uniform effectiveness), extra detergent must be added at regular intervals. In the past this has been done empirically by hand or by primitive means that involve constraints and disadvantages which do not, for example, permit addition of detergent following the evaluation of the viscosity or the pH of the bath nor rigorous sustaining of the detergent concentration in the bath, and which consequently do provide uniform washing effectiveness.